Surge relay circuit



Aug.2o,19ss 1T. LAMB 3,398,371 l suRG'E RELAY CIRCUIT i Filed dan. 4.]1966 y INVENTOR JOHN I.' LAMB ATTORNEYS United States Patent O 3,398,371 SURGE RELAY CIRCUIT John T. Lamb, Mansfield, Ohio, assignor to The Tappan Company, Mansfield, Ohio, a corporation of Ohio Filed Jan. 4, 1966, Ser. No. 518,626 4 Claims. (Cl. 328-9) ABsTRAT oF THE DISCLOSURE A circuit for applying full voltage to a magnetron microwave oscillator tube in steps consisting of a resistance element for limiting the application of voltage to a first level, and a'relay for sensing current buildup in the magnetron tube and for shunting the resistance element when a' predetermined current flows through the tube.

This invention relates to a power supply for a magnetron oscillator tube and more particularly, to delay and safety devices in the circuit.

The typical power supply for a magnetron oscillator tube is a heavy duty unit for supplying relatively high voltage and current levels to the tube to generate sufficient microwave energy for cooking, communication and varlous other purposes. It is the usual practice that these power supplies contain a heavy-duty transformer so that the high power levels may be supplied with good regulation providing relatively constant voltage and current across and through the magnetron during its operating period. Consequently, when the power supply is initially switched on, the transformer presents a very low impedance to the flow of current and an initial surge of current is experienced. This surge is not only reflected in the power line but also produces high transient voltages in the power supply which could be detrimental to the components in the supply and also to the'magnetron oscillator tube'. It is desirable that this surge bel eliminated or at least be reduced to alow level.

`In the past, it has been one practice in reducing the input surges upon switching on a power transformer to use a current-limiting` device in vseries with the transformer which is then cut out' ofl the circuit after a short period of time in order to apply full power to the transformer. One method has been to use a relay which is switched on at the same time as the transformer and whose contact is shunted across the current-limiting device. The relay closes its contact after a short interval vof time due to itsinherent time delay, which is the time for the relay to build up a sufficientfmagnetic field to pull in its contact-bearing armature. It is desired that this delay be at least eight milliseconds for a 60 c.p.s. power source and it is preferable that it be on the order of 20-30 milliseconds, this being sufficient time to set up asymmetrical flux in the core of the transformer so that it presents sufficient impedance to current ow. To obtain the necessary time delay, the relay must be larger and heavier than one rated for these voltage levels, necessitating an expensive, unwieldy and powerconsuming device.

An object of this invention, then, is to provide an improved power supply for a magnetron oscillator tube.

Another object of this invention is to provide an irnproved power supply for a magnetron oscillator tube, which power supply reduces the initial primary current surge through its power transformer.

A further object of this invention is to provide a power supply for a magnetron oscillator tube which power supply senses plate current flow through the magnetron oscillator tube and automatically supplies full power when suicient plate current is flowing.

A still further object of this invention is to provide a power supply for a magnetron oscillator tube, which ICC senses failure of the power supply to provide current, or failure of the magnetron oscillator tube to pass current and automatically removes input power to the power supply.

Other objects and advantages of the present invention willd become apparent as the following description procee s.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawing setting forth in detail an illustrative embodiment of the invention, this being indicative, however, of but one of the various ways in which the principles of the invention may be employed.

In said annexed drawing is a schematic circuit diagram of the power supply showing the element of the invention herein disclosed.

Refering now to the drawing, there is shown a transformer 10 whose input winding 11 is connected to a source 12 of alternating current power, and whose output winding 13 is connected to supply increased voltage to a magnetron oscillator tube 15. A rectifier circuit 18 converts the output of the transformer to direct current for application to the magnetron. The rectifier is of the standard bridgetype configuration, having four diodes and four junctions. The first 19 and second 20 junctions of the rectiler are connected to the secondary winding of the transformer and the third 2l and fourth 22 junctions provide a source of direct current across which the magnetron is connected. In series with the magnetron and the direct current source, is the coil 24 of a relay 25. The relay 25 is thus responsive to current flow through the magnetron 15 and upon receiving a predetermined quantity of current, is energized to close its contacts 26, 27. A current-limiting device 30 is located in series with the primary winding 11 of the transformer 10, an on-off power switch 32 and the alternating current power supply 12. The normally-open contacts 26, 27 of the relay are shunted across the current limiting device 30.

When the power switch 32 is closed, current flows through the primary windingll of the transformer ata reduced value due to the impedance of the current-lim1ting device 30. A relatively small voltage appears at the secondary 13 of the transformer and is converted by the rectifier 18 to cause current ow through the coil 24 of the relay and the magnetron 15. The reduced voltage applied to the transformer sets up a symmetrical ux in the core of the transformer 10, so that the transformer presents an increased impedance. Now full power may be applied to the transformer without creating a large surge of current which would draw heavily from the alternating current power source .12 and also produce a high transient voltage in the secondary 13 of the transformer, with possible destruction of components. The characteristics of the relay 25 are such that the relay is operated by the reduced value of current emanating from the rectifier 18 and passing through the magnetron 15, and closes its contacts 26, 27 to supply full power to the transformer 10. In actual practice, a small direct current relay may be used owing to the delay in the magnetron plate current build up, wherein a delay of twenty to thirty milliseconds may be obtained. The magnetron oscillator tube 15 is of the type well known in the art, which will, upon the application of suitable voltage between the anode 35 and cathode 36, and in the presence of a suitable inagnetic field, produce microwave energy. At these lower levels of voltage occurring within the delay interval, it is not important that the magnetron 15 produce the highfrequency energy. It is sufficient that the magnetron act similar to a conventional tube passing a quantity of current proportional to the voltage applied between its anode and cathode. When operating in this reduced voltage mode and due to the magnetic eld within the envelope of the tube opposing current flow, and the physical characteristics of the tube itself, actual current flow lags behind the applied voltage and provides the necessary time delay.

Variationscan be made in the extent of this delay by shunting a capacitance 40 across the coil 24 of the relay 25 as shown by dashed lines in the drawing. This capacitance 40 momentarily passes a small amount of current and acts as a shunt until the capacitance becomes charged, as is well understood in the art.

A smaller relay may be used if a resistance 42 in series with second contacts 46,- 47 of the relay 25V are shunted across the coil 24, also shown by dashed lines in the drawing. This resistance shunt will serve to divert the greater portion of the current once the relay 25 is energized Iand full power is supplied. This resistance shunt 42 will also affect the operational characteristics of the relay 25, causing it to drop out at a higher value of secondary current than when it was energized. However, this change in dropout level is not important since it 'is usual to maintain nearly full current in the magnetron when it is in an operational mode.

It may also be seen that the relay is a detector of failures of components in the power supply. Should one of the diodes in the rectifier 18 fail or if the secondary 13 of the transformer 10 opens or short-circuits, insufficient current will be produced in the circuit to energize the relay 25. Also, if the magnetron 15 fails to operate, that is, if the filament 50 is open circuited or if there is a misconnection, there will be no current flow from the rectifier 18. Thus, the relay 25 will fail to opcrate.

In this embodiment the current-limiting device is a resistor, although it may be any other device which limits current to the primary 11 of the transformer 10. An added feature of this invention is that the current-limiting device 30 may be of a lower current carrying capacity than the current being passed through it and the primary 11 of the transformer 10. In this arrangement, the current-limiting device 30 is being overloaded and is designed to fail or open-circuit if it remains in the circuit with the primary of the transformer for more than a short period of time. In the usual operation of the circuit the contacts 26, 27 of the relay will close before the currentlimiting device 30 fails and the contacts 26, 27 will carry the @full current applied to the primary 11 of t-he transformer 10. It can be seen then, that the current-limiting device 30 acts as a fuse with a built-in delay, and opens Iupon, any failure in the circuit that reduces or prevents plate current flow. The rating of the current limiting device 30 is usually of such a value that it limits primary current to a low enough amount so that no other component damage will occur.

It can be seen then, from the foregoing drawing and description, that we have provided a power supply for a magnetron oscillator tube, which has protection features inherent in its design. Initial current surges are automaticallyreduced and component or utilization device failures are automatically recognized and provided for.

Other modes of applying the principles of the invention may be amployed, change being made as lregards the details described, provided the features stated in any of the following claims or the equivalent of such be employed. A

I, therefore, particularly point out and distinctly claim as my invention: r

1. In combination with a magnetron high frequency oscillator tube, means for delaying the application of full power to the tube, saidvmeans .comprising a transformer having a primary circuit for receiving power from an alternating current source and a secondary circuit for applying-power to the magnetron tube, a fixed-'resistor serially connected in the primary circuit of the transformer` for limiting the application of power tothe magnetron tube t0 a predetermined level, rectifying means coupled across the secondary circuit of the transformer and having a pair of output terminals, relay means and said tube connected in a series'circuit across said output terminalsthe relay means being responsive -to the flow of current through the magnetron tu-be, and lbeing actuated when a predetermined level of current iiow is attained, a pair of contacts associated with the relay connected in normally open relationship across the fixed resistor, the contacts being closed upon actuation of the relay for shunting the fixed resistor to apply full power to the magnetron tube.

2. The combination set forth in claim 1 wherein the limiting means is a fixed resistor having a power rating `substantially lower than the power dissipated at the predetermined level such that failure of closure vof the pair of contacts in a normal time interval will cause failure of the resistor and an elimination of power -to the magnetron tube.

3. The combination set forth in claim 2 further including a second pair of normally open contacts associated with the relay, and a resistance element serially connected with the second pair of contacts across the coil of the relay to divert current flow from the relay coil when the relay is actuated.

4. The combination set forth in claim 3 further including a capacitor connected across the relay coil to shunt a portion of the -current flow through the relay to further detlaay the application of full power to the magnetron tu e.

References Cited vUNITED STATES PATENTS 3,181,039 4/1965v Binder et al. 317-154 3,317,699' 5/1967 Helfer 2l9-10.55 2,407,880 9/1946` Holtz 315-127 2,534,299 y12/1950 Rultz et al. 315-360 3,017,564- 1/1962 Barney 317-41 JOHN w. HUCKERT, Primary Examine. I. D. CRAIG, Assistant Examiner. 

